Your options depend on what sort of scam it is.
https://pages.ebay.com/securitycenter/index.html
Fraudulent listings can be reported using the "Report item" link in the listing itself.
If you are involved in a fraudulent transaction as a buyer or seller, there are links for those situations to report the seller or buyer.
If you received a message that is attempting to get you to violate policy or engage in an off eBay transaction, then most likely eBay will suspend the account before you can figure out how to contact eBay about it; scammers create new accounts or hijack existing accounts to send such messages, and when eBay suspends the account, the scammers just create new ones or hijack others.
If you saw a listing on another site and the seller claimed that eBay would handle the deal, report the listing on the site you found it on. eBay is not involved in listings on other sites.
